1. Field of the Invention
This invention relates to a method for making a treated polymer, more particularly to a method for making a treated polymer for a liquid crystal alignment agent. The invention also relates to a liquid crystal alignment agent containing the treated polymer, a liquid crystal alignment film formed from the liquid crystal alignment agent, and a liquid crystal display element including the liquid crystal alignment film.
2. Description of the Related Art
A liquid crystal display element is widely used in a screen of a computer, a viewfinder of a camera, a projection display, a television screen, or the like.
Nematic liquid crystal display elements are predominantly used in general liquid crystal display elements, and concrete examples of the nematic liquid crystal display elements actually used include: (1) a TN (Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at a 90 degrees angle relative to a liquid crystal alignment direction of the other side substrate; (2) a STN (Super Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at an angle greater than 180 degrees relative to a liquid crystal alignment direction of the other side substrate; and (3) a TFT (Thin Film Transistor) liquid crystal display element which uses a thin film transistor.
It has been continuously attempted in the art to improve a liquid crystal alignment film in order to enhance the display performance of the liquid crystal display elements, for example, to reduce the image sticking problem.
The liquid crystal alignment film is made of a liquid crystal alignment agent. Conventionally, the liquid crystal alignment agent is formulated by dissolving polyamic acid or soluble polyimide in an organic solvent, and is then applied and cured on a substrate to form the liquid crystal alignment film.
The image sticking problem results from the residual direct current (DC) voltage. Specifically, when the residual DC voltage is large, it will remain even after the electric field has been turned off after being once turned on. In such a case, images to be erased will remain as image sticking. Therefore, it is desired that the residual DC voltage approach zero infinitesimally. This image sticking phenomenon is one of the most important issues in liquid crystal display elements.
JP 11-193345 discloses a polyimide resin made of two or more polyamic acids having different properties for preparation of a liquid crystal alignment film.
WO 0061684 discloses a varnish composition comprising a polymer component comprising a polyamide acid having a specifically defined structure, a polyamide having a specifically defined structure, or the polyamide and a soluble polyimide having a specifically defined structure.
WO 2007078153 discloses a composition for liquid crystal alignment comprising an oligoimide or oligoamic acid, which includes a thermocurable or photocurable functional group on at least one end of the oligoimide or oligoamic acid backbone. It is described that in the optical resolution process, polyimide used for liquid crystal alignment is selectively photo-decomposed and inevitably leads to generate decomposed by-products in small units. Such decomposed by-products can cause very serious problems in alignment stability and long-term reliability, particularly image sticking.
US 2004/0031950 discloses a liquid crystal alignment agent. It is briefly described to use a poor solvent to purify a polyamic acid used for preparing the liquid crystal alignment agent.
It is still required in the art to develop a liquid crystal alignment agent which can effectively shorten an image sticking erasing time.